+P

Overpressure ammunition, commonly designated as +P or +P+, is small arms ammunition that has been loaded to a higher internal pressure than is standard for ammunition of its caliber (see internal ballistics), but less than the pressures generated by a proof round. This is done typically to produce rounds with a higher muzzle velocity and stopping power, such as ammunition used for defensive purposes. Because of this, +P ammunition is typically found in handgun calibers which might be used for defensive purposes.

Magnum cartridges, such as .357 Magnum or .44 Magnum, are usually developed by greatly increasing the working pressure of an existing cartridge, and the resulting cartridges are typically different in some small manner to prevent them from being chambered in firearms not specifically designed for them. For example, the .357 Magnum is slightly longer than the .38 Special, the lower powered round from which it was derived. +P ammunition, however, is externally identical to standard ammunition of its caliber. This was done when the ammunition manufacturers lowered the pressure of their standard rounds (1972). +P ammunition was designed to replicate the performance of what had been standard pressure cartridges. +P cartridges are not recommended to be used in firearms of questionable quality.

The burning characteristics of black powder used in early cartridges meant that these cartridges operated at low pressures, generally under 25,000 psi (~172,369 kPa). These cartridges were limited by their case capacity, and the only way to get more power was to increase the case to hold more powder; this can be seen in firearms such as those made by Sharps Rifle Manufacturing Company, which made rifles with nominal powder capacities from 70 grains (.45-70) to 110 grains (.45-110).

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